Well, there is no schematic for most trailers. You may have to pull each tail light and fish a wire, (the old fashioned way of wiring a harness). The green is the right turn/stop. The yellow is the left turn/stop, and the brown is parking lights. The white is ground. That is the normal color code. Are you sure towing double is legal where you are at?
Also having a hitch welded to the frame my void warranty, but your decision.
2012 Ford F250 ext. Cab 4x4
2002 Jeep Wrangler Sahara
2008 Yamaha V-Star 650 Classic
2008 Work and Play 18LT